(1) Field of the Invention
This invention relates generally to Liquid Crystal Displays (LCD), and more particularly to a method and a circuit to reduce power consumption of a LCD driver IC.
(2) Description of the Prior Art
Liquid crystal displays (LCD) use nematic liquid crystals. The molecular order in a nematic liquid crystal, which results from weak intermolecular forces, is easily disrupted. For this reason, the liquid crystals flow like an ordinary liquid. Because of the weakness of the intermolecular forces, the molecules in a nematic phase are easily realigned along new directions.
A liquid crystal display uses this ease of molecular reorientation to change areas of the display from light to dark, resulting in patterns that you see in the display. The display consists of liquid crystals contained between glass plates whose interior surfaces are treated to align the molecules in a given direction. When the voltage to a set of electrodes in some area of the display is turned on, the molecules of the liquid crystal in that area reorient along a new direction. When this voltage is turned off, the molecules return to their original orientation.
LCD's require an AC drive voltage with virtually no DC component. Prolonged DC operation may cause electrochemical reactions inside the display, which will cause significantly reduced life. It is essential that the voltage wave-form across the glass plates of the display be maintained at an average DC value of zero because the glass is likely to suffer a break-down if a non-zero DC voltage is applied for any sustained period of time. There is threshold behaviour for most LCD's and no change in transmission occurs until a threshold voltage, Vth, is reached. Transmission then decreases as the voltage increases until saturation is reached. Threshold voltage is typical 1.5–2.5 volts, and saturation occurs at about 4–5 volts.
The pixels across each horizontal “common” row of a LCD are connected together on the plate on one side of the liquid crystal film, and all the pixels in each vertical “segment” column are connected on the opposite side. The “commons” are then addressed serially by setting all the column voltages separately for each “common” and then turning on the “common” voltages in sequence.
Principally LCD's require a differential voltage greater than the threshold voltage Vth of the nematic fluid between two conducting layers to generate an “ON” pixel. The display consists of a matrix of pixels created by vertical “segment” (SEG) and horizontal “common” (COM) conductive layer either side of the nematic fluid. The display has the electrical characteristics of a capacitor, so requiring a “charging” current every time a “segment” and/or “common” are switched.
In order to display a whole picture the “commons” are scanned in sequence and the segments switched appropriately. This is done so that the applied root-mean-square (RMS) voltage between each common and segment is controlled to be greater (“ON”) or less than (“OFF”) the threshold voltage Vth of the display.
The data for the display is contained in a random access memory (RAM), which is typically structured to be the same as the display. For example, a display of 80 segments and 64 commons would have a RAM of 80 by 64 bits. The display scan reads a row of the RAM for each active common output.
Currently available driver IC's continually switch the LCD regardless of the displayed data. This causes a switching waveform, and hence power supply current, to be required through the whole display. This causes power consumption even if there is no change of data.
FIG. 1 prior art shows a simple display comprising 6 segments and 6 commons. The waveforms 2 show the sequencing of the commons over time during a display scan. The matrix 1 on the right shows the 36 pixels. The black rectangles represent “ON” pixels, the white rectangles represent “OFF” pixels. All the 6 commons are selected, independent if all pixels in a row are “OFF” or not. In the example of FIG. 1 prior art e.g. the rows 1, 5 and 6 are blank, this means all pixels are “OFF”. Nevertheless all commons got selected for sequencing of the pulses 2.
FIG. 2 prior art shows a basic circuit of a typical COM/SEG decode logic-generating signals to the pad control circuitry 22. Pad refers to the input to the LCD glass. The input to said SEG/COM decode logic 21 is data read out from a RAM and the PN signal. PN (Positive/Negative) refers to a signal to change the polarity between the Common and the Segment pad and is switching regularly between “0” and “1” to ensure that an average DC value of zero is achieved. The output of said SEG/COM decode logic 21 activates one of symmetric voltages V0–V3. Four transistors 23, 24, 25 and 26 perform the switching. The signal generated is linked to the related pad providing input to the LCD glass.
The following table shows related decode logic of said driver 21. The table shows which one of the four output voltages V0 to V3 is applied depending on the input values of data and PN:
DATAPNSEGCOM00V1V210V3V001V2V111V0V3For example, if the data is “1” either SEG or COM is at the maximum voltage V3, dependent on the value of the polarity signal PN and the related pixel is “lit”.
As another example of a typical implementation FIG. 3 prior art shows the COM and SEG voltage waveforms for a simple Super Twisted Nematic (STN) LCD display; where the outputs switch one of four symmetric voltages as it has been shown by FIG. 2 prior art. The curve 31 shows the waveform of the COMMON voltage, the curve 32 shows the waveform of the SEGMENT voltage. The related pixels 33 are lit (“1”) if either the COM signal or the SEG signal is at the maximum value V3.
U.S. patent (U.S. Pat. No. 5,825,343 to Moon) describes a driving device and a method of driving a TFT-LCD using a two-pulse electrode voltage to thereby double the duration of the driving impulse. The driving device includes a liquid crystal interface IC that outputs a two-pulse start signal and a clock signal. A gate bus driver IC outputs a two-pulse gate electrode voltage to each gate line according to the start signal inputted from the liquid crystal interface IC and a liquid crystal pixel is driven by the difference in potential between a grey voltage and a common electrode voltage.
U.S. patent (U.S. Pat. No. 5,986,631 to Nanno et al.) discloses a driving method of an active matrix LCD. According to this method, a scan signal has three voltages levels, i.e., an ON voltage, an OFF voltage and a compensation voltage having the opposite polarity with respect to the OFF voltage. In contrast with the conventional capacitively coupled driving method in which the scan signal consists of four voltages, the driving method of this invention can reduce a cost and power consumption for a driver IC without degradation due to flickers or other causes.
U.S. patent (U.S. Pat. No. 6,232,944 to Kumagawa et al.) shows a compact and inexpensive LCD by improving a drive method for compensating a cross-talk using a compensating pulse added to a signal voltage so that a drive IC and a periphery of the LCD panel are reduced in size. Only one of positive and negative compensating pulses is added in accordance with a predetermined period. The compensating pulse preferably has a waveform including low frequency components. A width or a height of the compensating pulse varies in accordance with a location of the signal electrode, display pattern or other factors.